Light-emitting diodes (LED) light sources, emitting either white light or colored light, are used for numerous applications such as interior and exterior lighting, decorative lighting, entertainment and the like. It is a recognized problem of the lighting industry that LED light sources must be carefully controlled in order to provide the best possible trade-off between requirements such as good electrical efficiency, high light intensity, color stability and color rendering.
In LED lighting systems having multiple LED emitters, the driving conditions of each emitter must be properly calibrated and controlled. Optimum driving conditions for each emitter must simultaneously take into account a specific target output color point as well as specific target light source intensity, and maintain both parameters stable over variations of environment temperature. In order to minimize the cost and complexity of a lighting system, it is desirable that appropriate LED driving conditions be obtained without resorting to color feedback, i.e., without measuring the light source emitted color, as this would require using expensive color sensors.
Identification of an appropriate drive condition for each LED emitter of a lighting system is nontrivial, since the color emitted by a LED emitter depends on the injected current and the LED junction temperature. As the LED dissipates heat when lit, the junction temperature is itself dependent on a number of parameters including the injected current, the junction voltage drop, the environment temperature and the efficiency at which the heat flowing from the junction to the environment is dissipated.
The drive condition of LED sources is often controlled by acting on the time-averaged forward current injected in the LED using some kind of current pulse modulation. A typical example is a PWM (Pulse Width Modulation) drive where the LED intensity is typically controlled by adjusting the duty cycle of a pulsed current waveform having constant predetermined maximum and minimum values (the latter being possibly set to zero). Various PWM schemes are known in the literature and may use a fixed or variable pulse frequency, constant or variable current values and complex waveforms. However, pulsed drive methods are affected by electromagnetic interference (EMI) problems and suffer from limitations on the achievable modulation depth. Furthermore, recent physiological studies demonstrate that slow PWM may create uncomfortable flickering of light. Minimizing the perceived flickering requires high frequency (>300 Hz) PWM, which may be hard and costly to implement.
PWM is nevertheless often chosen for LED driving as it is an energy efficient current modulation method. Furthermore, its implementation is relatively straightforward as the LED intensity is an approximately linear function of the PWM duty cycle.
Constant Current (CC) regulation is an alternative driving method that creates no flickering, low EMI and allows for larger variations of LED intensity. However, it can be energetically inefficient and cause large color variations.
Depending on the application, one method may be preferred over the other. Hence, it would be advantageous to provide a color control method which is applicable with all LED drivers, independently on the type of current control scheme.
Several methods based on simple linear models relating the junction temperature of each LED emitter to the emitted color are known in the art. However, these methods are effective only over a limited range of temperatures where the linear approximation is valid. Such methods may be inadequate for outdoor lighting subjected to largely-varying temperatures over the year. Furthermore, the temperature of a LED emitter can be highly dependent on the LED casing and the efficiency of the heat dissipation in the lighting source design. Dimming control adds to this problem as a LED dimmed to low intensity will experience a junction temperature near the environment temperature, while a fully lit LED may have a junction temperature many tens of degrees above the temperature of the environment.
The quality of the light generated by a LED lighting system affects the perceived colors of an illuminated scene: the color rendering property of a LED system is then another factor to be taken into account. Color rendering can be characterized using the CRI (color rendering index), which is a color rendering metric standardized by the CIE (Commission Internationale de l'Éclairage), or the CQS (Color Quality Scale), which is an alternative metric proposed by the NIST. For example, it is recognized in the literature that a CRI of at least 90 is desirable for lighting applications. The quality of color rendering is particularly meaningful for LED lighting systems that generate white light. A minimum of three primary colors are required for additive color synthesis of white light. LED lighting systems with only three LED emitters cannot provide white light with a CRI of at least 90. LED-based lighting systems having four or more LED emitters with different “primary” colors can be used to reach or to exceed the CRI threshold of 90 if appropriately controlled. However, one faces additional challenges when controlling LED lighting systems having more than three LED emitters with different “primary” colors. There remains a need in the field of high-end lighting applications employing additive color synthesis of white light for a LED control method capable of providing simultaneously a specified target white shade, a specified target intensity, and maximum color rendering as permitted by the controlled LED system.
The method should further allow maintaining these specified targets for the intensity and white shade over variations of environment temperature. Shades of white light are typically described by the light CCT (Correlated Color Temperature), but can also be described as a target color point in an appropriate color space.
There remains a need for a LED control method capable of maintaining a specified target light color over variations of light intensity (LED dimming) and environment temperature, without resorting to color feedback.
The known control methods rarely use the optimization of a color rendering metric as parameter for the control of LED emitters. In fact, the implementation of control methods that optimize the LED output according to a color rendering metric may be very demanding in terms of resources. The CRI color rendering metric (CRI) and many alternatives (such as the CQS) are based on a measurement of the light source spectrum. This requires the use of a spectrometer, a complex instrument that measures the light source spectrum. It is not convenient to include color feedback based on such an instrument as its cost is high.
Finally, there remains a need for a practical control method for operating a LED lighting system that can be effective over a large temperature range, and is applicable for various LED casing designs and for large dimming levels.